One of the byproducts of power plants and turbine engines is exhaust gas, commonly known as flue gas. This gas may contain components which are harmful to the environment, such as oxides of nitrogen (NOX). The production of NOX can occur when fossil fuels are combusted, such as in turbines, refinery heaters, steam boilers, etc. Such fuels include coal, oil, natural gas, waste product such as municipal solid waste, petroleum coke, and other carbon-based materials. It is beneficial to the environment to control the levels of NOX released into the atmosphere by burning such fuels.
One common approach to handling this situation is to inject a reducing gas into the flue gas stream, which will remove NOX, from the stream. One common method is a selective catalytic reduction (SCR) process which involves the injection of ammonia (NH3) into the flue gas stream and then passing the resultant flue gas over a catalyst. The flue gas and the ammonia regent travel through a catalytic converter that facilitates the breakdown of NOX into nitrogen (N2), oxygen (O2), and water, which are not harmful to the atmosphere.
One known method for generating ammonia vapor uses anhydrous ammonia, which is evaporated with either a direct electric heat source or with steam coils directly supplying the heat to the ammonia. The vaporized ammonia is then diluted with air in order to provide an adequate mass necessary to distribute the ammonia reagent evenly over a large ductwork cross-section. A disadvantage of such a system is the need for maintenance of the electric or steam coils as they are in direct contact with the anhydrous ammonia. Further, any breach of the steam coil could result in ammonia contamination of the steam system.
Another known method of generating ammonia vapor is to use a selective non-catalytic reduction process (SNCR) wherein a liquid aqueous ammonia derivative is sprayed into a high temperature region of the furnace in order to accomplish NOX reduction. In some systems the energy from the flue gas is used to accomplish the phase change. A major problem associated with this method of relying on the flue gas to supply the heat is that a long time is required in the hot region in order to vaporize the water and ammonia and for the reaction with the NOX.
Another known method is to vaporize aqueous ammonia, and then inject the vapor into the flue gas stream at a location upstream of the SCR reactor. A known approach to heating the aqueous ammonia is to use a diverted portion of flue gas, to heat up the aqueous ammonia. A problem with this approach is that everything that the flue gas comes in contact with is contaminated, from such contaminates in the flue gas as dust, ash, and sulfur oxides. In addition, the amount of heat available for vaporization is a function of the flue gas temperature making control of vaporization more difficult than where the heat source can be readily controlled to supply differing amounts of heat depending on sensed operating conditions.
An undesirable byproduct of some systems is waste water which includes significant amounts of ammonia. From a cost and environmental standpoint high concentrations of ammonia in waste water can be undesirable. Ammonia that is discharged represents wastage of a reactant which could have been put to good use in the flue gas stream. In addition, waste water with high ammonia concentrations can be harmful is discharged into the environment. Accordingly, additional processing may be required to treat waste water with high ammonia concentrations. Such treatment can be costly.
In view of the above discussion, it should be appreciated that it would be useful if improvements in systems which are used to vaporize aqueous ammonia could be made. In particular it would be desirable if reductions in the concentration in the amount of ammonia included in waste water discharged from the system could be reduced and/or efficiencies in the way heat is used to vaporize aqueous ammonia could be obtained as compared to known systems. Thus, it should be appreciated that there is a need for new and improved ways of preparing an ammonia reagent with a relatively low level of water for injection into the flue gas stream of a furnace in order to reduce the level of NOX in the flue gas, while producing residual, e.g., waste water, with relatively low ammonia concentrations and/or which is free of ammonia.